High-frequency induction heating system



Feb. 18, 1947. L. w. GREGORY ETAL 2,416,172

HIGH FREQUENCY INDUCTION HEATING SYSTEM 2 Sheets-Sheet l v 7 Filed April 27, 1945 3% g x a w 3 m a 4 INVENTORS. lat/1e)- h. Orepaly dad P040!) Miler/non. BY

ATTORN WITNESSES' I 44K Feb. 18, 1947. w, GREGORY ETAL 2,416,172

HIGH FREQUENCY INDUCTION HEATING SYSTEM Filed April 27, 1943 2 Sheets-Sheet 2 TM arm? 0 ATTORNEY Patented Feb. 18, 1947 HIGH-FREQUENCY INDUCTION HEATING SYSTEM Luther W. Gregory and Ralph N. Harmon, Baltimore, Md., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application April 27, 1943, Serial N0. 484,708

, 6 Claims.

This invention relates to heating systems utilizing high frequency electric currents and, more particularly, to the quick heating of materials by electromagnetic or electrostatic means of en-, ergy transfer.

The high frequency heating apparatus constructed in accordance with this invention is particularly adaptable for production requirements and possesses great flexibility for use with various types and sizes of materials to be heated.

A particular feature of the invention resides in the improvement of the electromagnetic and electrostatic energy transfer circuit to obtain higher eillciency for a given operation and increased current output.

Another feature of the invention is the flexibility whereby the current output may be preset for each type of specimen or material to be heated and may be varied within wide limits of operation.

An advantage of the invention is that simple means are provided for the adjustment of the heating current during operation whereby the rate of heating may be maintained at a desired constant value despite changes in the absorption properties of the material due to thermal agitation.

Another advantage or the system herein describedis that predetermined operating conditions may be maintained automatically during operation for materials of different sizes and physical properties.

Other features and advantages will be apparent from the following description of the invention, pointed out in particularity by the appended claims, and taken in connection with the accompanying drawings, in which:

Figure 1 is a schematic circuit arrangement of the induction heating oscillator and improved load coil circuit in accordance with the invention.

cuit for supplying the load current.

Fig. 3 is a circuit diagram showing a modification for automatic operation to compensate for changes occurring in the material under treatment.

Fig. 4 shows a circuit modification for electrostatic transfer of high frequency energy.

Fig. 5 illustrates by means of curves the increase of effective load circuit current above the oscillator tank current.

The invention is particularly useful in connection with high-frequency heating systems Fig. 2 shows a modification of the circuit utiliz-' ,ing-the capacity branch of the oscillator tank cir- 1 sirable, and uneconomical.

utilizing a. vacuum tube oscillator as, the source of energy. The improvement concerns the energy transfer circuit and provides for the placement of the heating inductance in inductive transfer, or the heating capacity in capacitive transfer of energy, in series with the main oscillator tank circuit. The energy transfer circuit is so proportioned as to offer a small series impedance in comparison with the tank circuit impedance whereby changes in the effective impedance of the transfer circuit will not substantially affect the main tank circuit. Moreover, means are provided for increasing the current in the transfer circuit without materially changing the circulating current of the main tank circuit. The current control means are also adjustable at will or automatically to compensate for losses due to change in the reactance of the transfer circuit upon electromagnetic or electrostatic association with the specimen to be heated.

In arrangement as heretofore known, it is customary to form part of the tank circuit as the energy transfer circuit for the material tobe heated. The tank circuit inductance coil is usually so arranged that it surrounds'the object to be heated. When an object is inserted in the coil, the impedance of the tank circuit is considerably altered, and this condition must be compensated by changing the tuning capacity ofthe oscillator. Operation in this manner is unde- In order to greatly reduce the deleterious effect produced by the insertion of the material, it is proposed, in inductive heating circuits in accordance with this invention, to use a heating coil which is only a small part of the total tank circuit. Inasmuch as the coil is of relatively low impedance with respect to the main tank circuit, when the object 7 to be heated is inserted, the reflected impedance will not influence the main tank circuit to such an extent as to seriously interfere with the operation thereof or that any compensation of the tank circuit be needed to restore normal operating conditions. Similarly in capacitive heat transfer circuits it is proposed to use a heating capacity which forms a small part of the total tank circuit capacity. When the effective capacitive reactance is altered due to the placement of the specimen to be heated in the electrostatic field, the balance of reactances in the main tank circuit will not be materially affected.

Referring to Fig. 1, the oscillator system chosen for exemplifying the invention is a simple triode oscillator with conventional inductive coupling between it output and input circuits. The in vention' is also applicable to different types of power oscillator with equal improvement of its operation. The vacuum tube I is supplied with the required operating potentials from a rectifier power supply fed from the commercial power line circuit to which is connected the terminals of theprimary winding 2 of the transformer 3. The secondary winding 4 is connected to the filament 6 of the tube l, and another secondary winding 6 energizes the filament I of the rectifier tube 8. The third secondary winding l connects between the center-tap of the winding 6 and through filter reactors H and radio frequency choke coil l2 to the anode l3 of the vacuum tube The current conductive path is completed through the rectifier to the filament 5 in that the anode I5 is connected to the center tap of the winding 4. The filter circuit also includes condensers l6 and Ill between terminals of the reactor II and the ground potential side of the system which also terminates at the center tap of the winding 4. The grid circuit of the tube I includes between the grid electrode l8 and the filament 5 the grid circuit inductance l9 and grid resistor in series therewith, the latter being bypassed by the condenser 2i. The tank circuit of the oscillator includes the tank circuit inductance 23 which is electromagnetically coupled to the grid circuit inductance l9 and the tank 'circuit capacity 2 3 effectively in parallel therewith. The high potential side of the tank circuit is coupled to the anode l3 by means of the coupling condenser 25.

The energy transfer circuit in the arrangement shown in Fig. l is of the inductive type including a load coil 26 in series with the tank circuit inductance 23. The connection of the load coil 26 is preferably between ground and the ground potential terminal of the tank circuit inductance 23. It is to be particularly noted that the load coil 26 is shunted by a variable condenser 21 to form what may be termed an antiresonant circuit. The function of this condenser is important in the operation of the system and will be explained in greater detail hereafter. A dial 28 may be attached to the condenser in order to have various reference points for the setting thereof. The construction of the load coil 26 may take various physical forms to suit particular requirements. Generally it comprises a coil having a number of turns on an insulating form, or it may be self-supporting in view of the fact that the windings are made of relatively heavy material, for instance, copper tubing.

The particular type of construction of the load. coil 26 has no bearing on the invention and it may be of any shape or size as long as it is so designed that the effective impedance thereof is small in comparison with that of the tank circuit inductance.

The load coil 26 as shown in Fig. 1 is in series between the tank circuit inductance 23 and ground. In certain cases the load coil 26 may be connected instead in series between the capacitive branch of the tank circuit and ground as shown in Fig. 2. The elements and components of the system are identically the same as in Fig; 1 and similar reference characters are used to identify them. The difference is only in the circuit placement of the coil 26 which is in series with the ground returnof the tank circuit capacity 24. Efiectively, the coil 26 is in series here also with the tank circuit in that the tank circuit current will flow through the coil 26 and contribute to the heating effect produced by r 4 the total current circulating in the energy transfer circuit. The latter comprises the coil 26 and the condenser 21.

Referring to the operation of the system described in connection with Figs. 1 or 2, the effective output of the oscillator tube l for heating purposes is produced by the current which flows through the impedance element associated with the material. namely, the coil 26. The latter being effectively in series with the main tank circuit will carry the tank circuit current. This in most cases is of sufiicient intensity to produce the required heating effect on the material. However, as the material is inserted, there will be a change of the current due to the change of the effective reactance of the coil 26. It was mentioned before that the load coil is so proportioned as to have a comparatively small reactance value with respect to the main tank circuit inductance 23. Consequently, the current change produced by the material will have small effect on the tank circuit and the operation of the oscillator will not change materially. While this effect is small it can be further reduced and at the same time the heating current increased to a value several times that of the tank circuit current by shunting the heating coil 26 with a suitable condenser 21 as shown in Figs. 1 and 2. The function of this condenser is one of the salient points of the invention and lends not only higher efliciency to the system but also great flexibility in heating current control without interfering with the main oscillator circuit. When the condenser 2'! has the proper capacitance, forming a parallel resonant circuit with the coil '26, at the operating frequency, the effective current will be increased. Theoretically the current would tend to be infinitely high and the practical value of it is governed by the resistive or PR losses of the coil. Obviously, by varying the capacity of the condenser 21, the effective current may be varied to a suitable or desired value for the particular material to be heated. Irrespective of the material under the electromagnetic influence of the coil 26, the effective current in the coil will be increased by the use of the condenser 21 to a value above the tank circuit current which would normally flow through it. This increase is shown by the curves in Fig. 5 in which the ordinates show the ratio of inductive to capacitive reactance of the coil 26 and the condenser 21. For'the sake of illustration this ratio is taken as unity at the maximum operating current. The abscissae show the relative increase in the effective heating current above the tank circuit current which is also chosen as unity. The current increase from the value shown by curve A to that of B and C indicates the conditions obtained when the electromagnetic coupling between the load coil and the material is changed. In other words, the current changes with a particular setting of the condenser 27 when different types of materials are inserted in the coil 26. For a material which takes up a larger amount of space, let us say a steel rod of'large diameter, the current will increase above the tank current to the value shown in curve A. With a material of small diameter giving less magnetic coupling, the current will increase to the value shown in curve B. Whereas, with a material producing less change in the reactance of the coil 26, the current will be higher, as shown in curve C.

The current above the tank circuit current due to the condenser 21 is..therefore, limited only by the inherent resistive loss of the coil 23. Any change in thi condition which will produce a greater resistance loss will lower the current in the direction approaching the value of the normal tank current. It is clear from the above that with a particular setting, of the condenser 21 the material under treatment will change the effective heating current. Conversely, with a certain material to be treated the effective current can be varied by changing the capacity of the condenser 21.

When the heating system is used for production operation of a certain specified material, the desired current can be preset by adustment of the condenser 21, to a predetermined point on a suitable dial or indicating device 28. When different sizes of materials are to be treated, the dial may be placed to the particular reference point used for the new material. It is also often advantageous to change the setting of the condenser while the material is under treatment to maintain uniform heating effect. Particularly to compensate for the reactive change caused by the change in magnetic properties of the material when it reaches its Curie point.

A system which permits automatic setting of the heating current in accordance with the changes produced by the thermal eflect on the material is shownin Fig. 3. Identical components with those shown in Fig. 1 are marked with similar reference characters. The condenser 21 is shown to be actuated by a motor 34 which has two separate field windings and 38 in series with the armature 31. The operating source for the motor 34 is derived from an additional secondary winding 38 of the transformer 3. The output thereof is rectified by a conventional bridge circuit arrangement showing rectiflers 38 supplying direct current to a load resistor 48. One terminal of resistor 48 is connected to the armature 31 and the other terminal to a single pole double throw switch forming the movement of a polarized relay. When the moving contact 41 of the relay 42 engages the contact 43, the field 38 is energized, and the motor will rotate in one direction, whereas when the moving contact 4| engages the contact 44, the field 35 will be energized, and the motor will turn in the other direction. The movement of the relay 42 is effected by the relay coil 48, one terminal of which connects to a variable tap of the resistor 48, and the other terminal to ground potential of the oscillator system. The oscillator circuit, in addition to the conventional components described in connection with Fig. 1, includes also a resistor 41 which is in series with the anode-cathode circuit, carrying thereby the anode current. The positive terminal of the rectifier powersupply source connects to the filament terminal of the resistor 41. The anode current will produce a voltage drop across the resistor 41, and in this respect the latter may beregarded as a voltage source. Upon current flow. the voltage drop will be of such polarity that the negative terminal will be the one which connects to ground. In other words, the voltage produced across the resistor 41 and the voltage supplied across the resistor will oppose each other. The rider of the resistor 48 may be adiusted under static or normal operating conditions to a point where the two voltages are equal and the coil 48 is not energized. When no current fiows through the coil 48, moving contact 41 is held in a neutral position between the con tacts 43 and 44 by the springs III and 8|. The

duced across resistor 41 small change in the oscillator circuit due to a reactance change produced by the material is sufficient to unbalance the voltage relations above referred to the oscillator tube will also change. and the voltage produced across resistor 41 will be of a higher or a lower value than before. The two voltages previously balanced between'terminals of the coil 48 will be unbalanced, and one will predominate over the other. If the voltage proincreases, the current will fiow in one direction to the coil 48, causing movement of the contact 41. The connection of the fields 38 and 38, respectively, are so made to the contacts 43 and 44 that when the current is increased in the resistor 41, the motor will turn the condenser 21 in a proper direction to restore the previous current equilibrium. The motor will turn until the voltages across resistors 41 and 48 are balanced out. Inversely, when the voltage drop across the resistor 41 decreases, the current will fiow in the opposite direction through the coil 46, causing the motor to reverse and actuate the condenser 21 to the point where the voltage drop across the resistor 41 will be increased to balance that across the resistor 40.

Referring to Fig. 4 the modification of the highfrequency heating circuit herein shown discloses the capacitive energy transfer for heating materials having no magnetic properties. Among such materials, as a practical example, may be mentioned tobacco which can be very efllciently drie by high-frequency treatment. The advantages offered by this method of treatment are manifold and proved practical in the tobacco industry.

The material to be heated is placed in the electrostatic field of the condenser 21 and becomes a dielectric body within the electrostatic field. To obtain the advantages offered by the induction heating circuits shown above as to increased heating current and ease of control, the condenser 21 is shunted by the inductance 28 which may be made variable as for example by a series of taps connected to arotary switch 80 or in any desired manner. he dial 28 is here associated with the'switch 38 to indicate the particular tap selected.

The operation of this circuit is essentially the same as described in connection withthe induction heating circuits except that the tuning to control the current in the energy transfer circuit is effected by varying the inductance 28 instead of the capacity 21. The latter is the fixed element and the actual load circuit. The curves shown in Fig. 5 will apply also to the operating conditions of the circuit of Fig. 4.

The improvements tion are twofold. The low impedance energy transfer circuit placed in series with the tank circuit of the oscillator eliminates the deleterious effect of reflected resistance in the oscillator tank circuit which would be accompanied by changes in the .heating current. Therefore the heating current remains substantially constant since the tank circuit current will-not vary. In addition to that the heating current can be increased above the value of the tank circuit current by simple means resulting in an eflective current control over wide limits.

We claim as our invention:

1. In combination .with means for producing alternating current, a network energized from said means and comprising capacitance. inductance and an anti-resonant circuit all conin that the anode current of contributed by this inven-.

nected in series, a portion of said anti-resonant circuit comprising means for transferring electromagnetic energy to a load undergoing heat treatment.

2. In combination with means for producing high-frequency alternating current, a network energized from said means and comprising capacitance. inductance and an anti-resonant circuit all connected in series, one branch of said anti-resonant circuit comprising means for transferring electromagnetic energy to a load undergoing heat treatment.

3. In combination with means for producing alternating current, a network energized from said means and comprising capacitance, inductance and an anti-resonant circuit all connected in series, one branch of said anti-resonant circuit comprising means for transferring electromagnetic energy to a load undergoing heat treatment, the reactance of said one branch being small compared to the reactance of said inductance at the frequency of said alternating current.

4. In combination with means for producing alternating current, a network energized from said means and comprising capacitance, induct ance and an anti-resonant circuit all connected in series, one branch of said anti-resonant circuit comprising.means for transferring electromagnetic energy to a load undergoing heat treatment, and means responsive to current flo'ving through said source to vary the reactance of the other branch of said anti-resonant circuit.

5. A work circuit having an induction heating system comprising an inductance, a capacitance, and an anti-resonant circuit all serially connected with each other, a portion of said anti-resonant circuit being adapted to be inductively related to a load to be heated to thereby receive electromagnetic energy therefrom.

6. A work circuit having an induction heating system comprising an inductance, a capacitance, and an anti-resonant circuit all serially connected with each other, one branch of said anti-resonant 5 circuit being adapted to be inductively related to a load to be heated to thereby receive electromagnetic energy therefrom and the other branch of said anti-resonant circuit being adjustable.

LUTHER W. GREGORY. l RALPH N. HARMON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,719,521 Schafi'er July 2, 1929 2,013,806 Osnos Sept. 10, 1935 1,551,822 Grant Sept. 1, 1925 2,251,277 Hart Aug. 5, 1941 1,833,617 Northrup Nov. 24, 1931 1,604,981 Elsasser Nov. 2, 1926 2,313,440 Huge Mar. 9, 1943 2,101,715 Klotz et a1 Dec. 7, 1937 2,351,604 Ferrill June 20, 1944 2,384,799 Cook Sept. 18, 1945 FOREIGN PATENTS y Number Country Date 410,182 British 1933 457,381 British Nov. 26, 1936 461,289 British Feb. 15, 1937 8,154 Australian Dec. 13, 1932 OTHER REFERENCES Humphrey Electronic Generators Extend Induction Heating Field, Electronics, J an., 1943. page 58. (Copy in Scientific Library.) 

